Mature two-spotted spider mites are ~1/32" long, with two well-defined spots near the front of the body. Two spotted spider mites are somewhat larger and more robust than Banks grass mites.

Mites destroy individual cells on undersides of leaves, causing yellow spots; in severe cases leaf death occurs. Two-spotted spider mites tend to occur over the entire plant as populations increase.

May be present throughout the growing season, particularly in drought-stressed field areas. Most common in July, August, and into September (statewide).

Examine the undersides of leaves of several plants in several different parts of the field. Note the mite species present, the distribution of colonies, and the amount of injury.

For two-spotted spider mites only or two-spotted spider mite and banks grass mite: 15 - 20% of the total leaf area with active mite colonies and moderate damage apparent. Economic threshold table in NebGuide G1167.

http://www.unl.edu/entomology/extension/crops/fieldcorn.shtml